The subject matter of this invention is related generally to timing circuits for circuit breaker control systems and is related more particularly to those circuits which protect the circuit breaker from catastrophic failure due to an excessively high amount of fault current.
The use of various kinds of timing circuits in the control systems for circuit interrupters is well known. The various timing circuits sense predetermined levels of fault currents or overload currents and cause a timed or instantaneous opening of the circuit breaker contacts to protect the circuit which is to be protected. Apparatus of the type previously described is disclosed in U.S. Pat. No. 3,634,729, issued Jan. 11, 1972, to A. J. Hendry and J. T. Wilson and assigned to the assignee of the present invention. The prior art teaches or provides three well-known ranges of tripping. First, long delay tripping circuits are provided which actuate the tripping of the circuit breaker in inverse proportion to the square of the overload current. Second, short delay tripping circuits are taught which actuate the circuit breaker apparatus after a fixed delay when a predetermined relatively higher value of current, such as fault current, has been attained. Third, instantaneous trip circuits are provided which cause the circuit breaker apparatus to trip generally instantaneously when catastrophically high values of fault current occur in the line to be protected. Generally the timing circuits are provided to optimize the relationship between safety and convenience; that is, it is desirous to trip a circuit breaker only when it is absolutely necessary to so do and not before. Generally the instantaneous tripping characteristic of a circuit breaker is at least as much related to the physical composition of the circuit breaker apparatus as it is to the circuit to be protected. Said in another way, there are some values of fault current which are not only catastrophic in terms of the circuit to be protected, but which if allowed to continue to flow through the circuit breaker contacts, would physically destroy the circuit breaker by destroying the contacts thereof or by causing an explosion therein. The capability of an uncontrolled circuit breaker apparatus to withstand the energy of a fault is not without limit. However, even in the extreme case, the inconvenience of opening a circuit breaker and thus shutting down an entire electrical network or line is such that it is desirous to prevent the circuit breaker from tripping if the extreme fault disappears or is corrected elsewhere very quickly. Consequently, it would be advantageous if a circuit breaker protective timing circuit could be provided which on the one hand senses extreme values of current of a sufficiently high magnitude to destroy or otherwise seriously damage the circuit breaker if not interrupted very quickly, while on the other hand, introduces a slight delay in the opening of the circuit breaker contacts if the value of fault current can nevertheless be tolerated for that slight period of time to therefore check the status of the fault at a later time within the slight delay period to verify the need for tripping and abort it if possible.